Imposition column absorbs dragged columns

[thinkh]

Steps to reproduce

1. Add imposition column
2. Drop numerical column
3. Drop categorical column (up to here it's working)
4. Drop another numerical column
5. Drop another categorical column

Observed behavior

Dropping further numerical or categorical columns results in the loss of the column. The user has to add the columns manually again.

Expected behavior

There are two possible behaviors (we have to discuss and decide):

First option

The last dropped column overrides the previous column (which is not the case at the moment). Then it would be nice if the previous column will be added next to the imposition column, to prevent that the user has to add it again from the add column dropdown.

Second option

The last dropped column gets rejected by the imposition column and appears at its original position. However, this might be a bit counter-intuitive, because then the user has to split the imposition column first and add a new imposition column from the add column dropdown.

[thinkh]

General issue -> moved to lineupjs#114.